A petroleum reservoir is typically a stratum of rock containing interconnected pore spaces which are saturated with oil, water, and/or gas. Knowledge of the relative amounts of these fluids in the formation, and residual oil saturation (ROS) in particular, is essential for planning of processes to recover oil from the reservoir and for estimating economic incentives to proceed with these processes.
Several methods have been utilized to determine ROS in oil reservoirs. Direct measurements may be obtained from a core sample. This can be very expensive. Additionally, there is also a high likelihood that the drilling required to obtain the sample will alter the fluid contents of the sample. Finally, coring only obtains information on fluid saturations at the point the core sample is obtained. Different regions of the reservoir have likely been exposed to recovery methods having a varying effectiveness.
Indications of ROS may also be obtained from well logs. This is relatively simple and inexpensive, but the data obtained are of limited accuracy. Data are only obtained for the formation that is within a few feet of the well. This portion of the formation has likely been disproportionally affected by production from the well. The logs also, by their nature, measure properties of the rock-fluid system. It is difficult to differentiate between the properties of the various fluid phases within the rock. The resultant estimates of ROS can therefore have significant errors.
ROS can also be approximated by performing material balances based on initial oil saturation and production histories. These estimates are subject to significant errors due to the dependence on knowledge of the amount of oil initially within the reservoir, and the difficulty of measuring and tracking accumulated oil and gas production.
In U.S. Pat. No. 3,623,842, a method to determine ROS is disclosed in which a hydrolyzable component in a carrier fluid is injected into the formation through a wellbore. This type of method is commonly referred to as a single well tracer test (SWTT). The component is one which partitions into both the oil and the water phases, and therefore propagates through the formation in a rate which is lower than the rate which the carrier fluids travel into the formation. The component is then permitted to remain in the formation for a "soak" period during which the component partially hydrolyzes, forming a product which is water soluble. After a portion, but not all, of the hydrolyzable component is hydrolyzed, fluids are produced from the well. The concentration of the hydrolyzable component and the water soluble hydrolysis product in the produced fluids are tracked. The product of the hydrolyzation will be produced with the produced fluid which was in the formation at the location of the hydrolyzable component during the soak period. The hydrolyzable component is not produced at that time because propagation back through the formation toward the well is delayed due to the partitioning between the oil and the water phases. The volume produced before the hydrolyzable component is produced, and the volume produced before the product of the hydrolysis is produced, along with the partitioning coefficients for each are then used to calculate the ratio of the two phases in the formation.
The method of '842 has been useful in many applications, but has shortcomings. The hydrolysis must consume a portion of the hydrolyzable component which results in measureable concentrations of both the hydrolyzable component and the product. The propagation of the hydrolyzable component into the formation is retarded due to partitioning between the mobile and the immobile phase. It would be preferred to have a component which is soluble only in the mobile phase injected into the formation because the ROS could then be determined at locations farther from the injection well. The hydrolyzable component must be one which does not significantly hydrolyze during the injection phase, but partially hydrolyzes during a soak period of a reasonable length under reservoir conditions. Hydrolysis of the potential hydrolyzable materials is very dependent upon conditions such as temperature and pH. These conditions can vary significantly from reservoir to reservoir. Suitable components are therefore dependent upon the particular reservoirs. Considerable planning and modeling must therefore go into ROS determination by this method.
A single-well tracer method which eliminates many of the problems of the method disclosed in '842 is disclosed in U.S. Pat. No. 3,856,468. This patent discloses injection of two water-soluble components in an aqueous carrier fluid into a well. One of the components is a precursor which reacts within the formation to form a partitionable tracer. The other component remains as a water-soluble tracer. After the components are injected and driven into the formation, they are allowed to soak. During this soak, the component which reacts to form the partitioning tracer does so. This method does not rely on a limited portion of the reactive component forming the tracer. Precursors can therefore be used in a wider variety of reservoir conditions. This method also eliminates the errors caused by the hydrolysis of the precursor occurring in the production phase. Additionally, the reservoir is tested farther from the well due to water soluble components being injected. This process therefore overcomes many of the shortcomings of the process disclosed in '842.
Although the components of patent '468 are said to be water soluble, they do have some oil solubility and may undergo some chromatographic separation in the process of being transported into the formation. Being different components, chromatographic separation differs for the different tracers. These things could cause the tracers to come back to the production well from different locations of the reservoir and would be undesirable.
Determination of ROS using the SWTT of patents '482 and '468 is also subject to considerable errors due to problems arising from interpretation of the data. The amount of tracers detected in the fluids produced from the well depends not only on the relative amount of oil, but also on drift, transient reactions, fluid loss and irreversible flow from the wellbore vicinity. When various combinations of these factors could be significant, a unique value for ROS cannot be determined due to the large number of variables which must be inferred in interpretation of the well test data. Tomich et al., in "Single-Well Tracer Method to Measure Residual Oil Saturation," Journal of Petroleum Technology, pp. 211-18, February 1973, discuss the effect of drift on the determination of ROS by SWTTs. A small scale mini-test is utilized to obtain better estimates of drift and reaction rate for use in designing a main test. The mini-test involves injection of a limited amount of ethyl acetate, allowing the ethyl acetate to partially hydrolyze in the formation, and then producing from the well. The limited amount of tracer injected prevents the mini-test from permitting determination of ROS with acceptable accuracy. Tomich claims that the mini-test is used to fine tune the test system and to estimate drift. But drift is estimated from the mini-test in the same way it is estimated from the main test. The mini-test, performed as Tomich performs the mini-test, only gives a second determination of drift. It does not yield an improved or more accurate measurement of drift.
Sheely, in "Description of Field Tests to Determine Residual Oil Saturation by Single-Well Tracer Method," Journal of Petroleum Technology, pp. 194-202, February 1978, discloses the incorporation of "irreversible flow" in accounting for the SWTT results. Drift and ROS could not account for the concentration profile of the tracer produced in a particular application. Sheely concludes that the SWTT can be useful, but is subject to considerable data interpretation. Applicants have further found that such data interpretation can result in multiple interpretations which each indicate a different ROS. It is preferable to have a method to determine ROS which does not rely on such data interpretation.
Determination of drift, fluid loss from the formation, irreversible flow from the formation have independent significance. Drift indicates the velocity at which fluids are moving in a formation. Such information is useful in planning locations of new wells and workovers of existing wells. Fluid loss from the formation and irreversible flow from the formation may indicate that fluids are communicating between layers in the formations. Such communications may be through wellbores which are improperly cemented. Knowing that this fluid loss is occurring could indicate that further remediation work is needed.
Prior art SWTTs which incorporate a hydrolyzable ester to generate tracers in-situ also suffer from hydrolyzation while the esters are in transit into the formation. Hydrolyzation rates are very pH dependent, being much faster at pHs above about 4 or 5. Because the generation of acid in-situ lowers the pH, rapid hydrolyzation can occur while the tracer precursors are being transmitted into the formation. This hydrolyzation is neglected by the prior art under the rationale that the time period is short compared to the soak. Neglecting these transient reactions introduces another source of error in interpretation of the single well tracer test data.
It is therefore an object of the present invention to provide a process to determine residual oil saturation or drift of a subterranean formation in the vicinity of a wellbore. In a preferred embodiment it is an object to provide such a process in which a water soluble tracer precursor is injected into the formation and within the formation transforms into a water-soluble tracer and a partitioning tracer.